For example, as described in Patent Literature 1, there is hitherto known a keyboard instrument with a soundboard, which is configured so that an electric signal representing a music sound generated from a sound source circuit in accordance with keyboard performance is introduced to a coil of a transducer for vibrating the soundboard, and the soundboard is vibrated in accordance with the electric signal representing the music sound to generate a small instrumental sound.
Further, in Patent Literature 2, there is disclosed a technology in which a transformer having a primary winding that is arranged between an output amplifier and a speaker and is coupled to a high frequency coil in a switching power supply circuit detects an abnormal current so that the speaker and the output amplifier are protected via a CPU.